Radioactive Iodine FAQ

My cat was just diagnosed with hyperthyroidism. My vet says we should give her methimazole. What should I know?

Methimazole was developed for human use in hyperthyroid patients who were diagnosed during pregnancy, on a short waiting list for radioactive iodine treatment, or terminally ill. It was never intended for long term use, not even in humans. Methimazole does not stop the growth of the thyroid tumor, has a short drug half-life of effectiveness, and has an extremely high rate of drug reactions. Over 20% of patients cannot tolerate this medication. As the tumor grows and the level of thyroid hormones rises, progressive damage to all organs of the body dramatically increases. The heart and kidneys take the biggest hits from hyperthyroidism, so the earlier the disease is diagnosed and cured, the better the outcome for the patient. Radio-iodine in human and in veterinary medicine is the gold standard of care, and all other therapies are much less effective and/or dangerous. Studies have shown that radio-iodine patients live 2-3 times longer than methimazole treated patients, while the overall costs are 2-4 times higher for methimazole treated patients.

What tests does my cat need prior to treatment?

A current blood chemistry profile, blood pressure, and a urinalysis with specific gravity by refractometer are needed. The blood chemistry must include a T4 or T4 and Free T4 (for borderline cases) which is run at a reference (outside) lab. This result typically takes about two days. Most practices will run the urinalysis in house. If your veterinarian has referred to us before, he or she is already very familiar with these requirements. And if your veterinarian is the one referring your cat, it is 99% likely that these tests have already been completed. My cat is indoor only and has not been vaccinated in many years. Do you require any vaccinations?

All rabies vaccinations need to be current as mandated by the laws of Colorado. We also require that patients be current on feline distemper and the upper respiratory vaccines for herpes and calici viruses. Vaccinations are for the protection of our staff (rabies) and for all our feline patients.

My cat has abnormal liver values on blood chemistry. Should she go to an internist before radioactive iodine treatment?

Hyperthyroidism as it progresses causes the liver to be overworked and become damaged. Additionally, 20% of patients who have received methimazole become sick and have rising liver enzymes. Most elevated liver enzymes are associated with the rise in thyroid hormones, and go back to normal after radio-iodine treatment. It is uncommon in hyperthyroid patients to have primary liver disease. Separating primary and secondary liver disease is extremely difficult. The odds favor curing the hyperthyroidism and having both diseases resolve. If a primary liver problem remains, it should be worked up as a separate, underlying disease.

Why is it so expensive?

Hyperthyroidism is a major endocrine disease that can be cured. Radio-iodine therapy is the gold standard in human and in veterinary medicine, and is very cost effective. Compare the costs and continued costs of medically treated hyperthyroidism, thyroid surgery, twice daily insulin treatments for diabetes, fractured limb repairs, treating congestive heart failure, treating chronic liver and kidney disease and then compare the curing of hyperthyroidism. The cost to value ratio is amazing. When was the last time you heard the word “cure” used with any major, chronic medical condition?

How is radio-iodine administered?

It is given as a subcutaneous injection, similar to a vaccine. It is not injected into the thyroid gland. Within 20 minutes the radio-iodine dose is being absorbed into the bloodstream and is targeting the thyroid gland in the neck. Active thyroid tissue is the only tissue that concentrates radio-iodine. No other tissue is affected.

When do you do the treatments?

On Mondays, so that all our patients can go home the same week. We can do the treatment on a different day, at your request, but we do charge an extra fee for that because our nuclear pharmacy must make a special delivery to us. Please call us for the details if that need exists.

Why will hyperthyroidism ultimately kill my cat?

Hyperthyroidism is a multi-systemic disease that affects all organs of the body – from the brain to the tip of the tail and everything in between. It severely damages the heart, kidneys and liver by making these organs work harder and work overtime. The body can’t rest, and a negative metabolic state becomes normal. The end result can be total heart failure, kidney failure, liver failure, and the collapse of all systems. It is a common misconception that methimazole will stop this. Its effect is partial at best. Methimazole masks the symptoms of hyperthyroidism, while the tumor continues to grow and the disease continues to advance. Dosage of methimazole needs to be continually monitored. It must be increased or in some cases decreased or stopped as the disease continues to advance until many cats cannot be controlled with medication. When the cats become intolerant to methimazole, these patients are sick and can only be saved with radio-iodine. These are very high risk patients with questionable outcomes. Treat early for a cure!

Why does the treatment take so long?

The treatment itself takes about a minute. What you are referring to is the State of Colorado mandate that your cat must stay with us in our nuclear medicine ward for a few days until levels of radioiodine in their bodies fall below State guidelines. Most cats go home on the third day; higher dose cats will stay longer.

What is my cat’s stay at Cat Specialist like?

Our nuclear medicine ward was designed for extended stay, featuring a private ward with oversized cages complete with second story balconies and soft kitty beds. The room is filled with natural light from skylights, and our negative pressure HVAC system constantly exhausts room air and replaces it with fresh air. We do not spend extra time with the patients because it is important to limit staff exposure. But we serve 2 (3 if indicated) meals each day, clean litterboxes, and we are allowed to play with the cats while we are doing our chores. We are allowed to administer any medications your cat needs. Most cats settle in very quickly and seem to really enjoy our scheduled visits. Remember, our nurses all love cats!

How many treatments are involved?

96% of radio-iodine treated cats cured with one dose. A few need a second dose to treat the uncommon cancer form of the disease.

Will this make my cat sick?

No. Radio-iodine specifically concentrates only in thyroid tissue and targets the active tumor cells. No other tissue in the body is affected by the radio-iodine. This treatment is very different from traditional radiation therapy or chemotherapy which affect the whole body. Cats also get very small doses compared to humans, –commonly 50-100 times less.

Will my cat have to take medication after the treatment?

Maybe. If the hyperthyroidism is cured (96%) most patients do not need medication. If the tumor mass was large, then thyroid supplementation with tablet L-thyroxine may be needed to add to the body’s thyroid hormone level (5%). If other underlying diseases are present, then their treatment will be needed.

What are the risks to my cat?

Radio-iodine at therapeutic doses has no known side-effects because the radio-iodine concentrates only in the thyroid tissue. Occasionally, high dose cats may exhibit signs of a sore throat or a hoarse meow for a few days following treatment. Cats appear to be very resistant to the effects of radiation compared to humans (5 to 10 times).

What are the risks to my family when my cat comes home?

The reason we keep your cat for 3 days following treatment is to eliminate such risks. Most of the radioactive iodine has been eliminated from your cat’s system when he or she goes home. You will need to limit close or extended contact with your cat for two weeks. This means snuggling and sleeping together and the cat cannot sleep in your bedroom. You can have as much play time together at arm’s length as you wish, only close cuddling is restricted. You will be asked to use flushable litter (we have it available). If your cat will not use flushable litter, then we will teach you how to use non-flushable litter and store the waste until it is safe to discard. If you can’t manage the simple release restrictions, we are very glad to board your cat for this brief time. Your cat has received an extremely small dose of radioactive iodine. Human patients receive 50 to 100 times the dose and they work beside you or sit on the bus or plane beside you, the very next day!

Why is there a problem with radio-iodine if I am pregnant?

There are additional restrictions about handling your cat if you are pregnant, but the restrictions can be easily managed. This is because your unborn baby is more susceptible to the effects of radiation than you are. There is no need to let your cat suffer from hyperthyroidism. Just go ahead and treat the cat, and then let us board him or her for the two week restrictive period following release. Just that quickly, your routine returns to normal.

When my cat is cured of hyperthyroidism, will all his/her other medical problems be cured?

Unfortunately, no. Secondary organ damage that was caused by hyperthyroidism will stop and some healing will take place. Primary and age related diseases will need to be treated as separate medical conditions (heart, liver, kidneys). Removing hyperthyroidism will make treating other diseases easier. Age brings with it many treatable chronic diseases. Cure the one early that has the potential to kill your cat: hyperthyroidism.